1. Field of the Invention
The present invention relates to light emitting devices. In particular, the present invention relates to a filter design for organic light emitting diode (“OLED”) devices.
2. Description of the Related Art
An OLED device typically includes a stack of thin layers formed on a substrate. In the stack, a light-emitting layer of a luminescent organic solid, as well as adjacent semiconductor layers, is sandwiched between a cathode and an anode. The light-emitting layer may be selected from any of a multitude of fluorescent organic solids. Any of the layers, and particularly the light-emitting layer, may consist of multiple sub layers.
In a typical OLED, either the cathode or the anode is transparent. The films may be formed by evaporation, spin casting, other appropriate polymer film-forming techniques, or chemical self-assembly. Thicknesses typically range from a few monolayers to about 1 to 2,000 angstroms. Protection of OLED against oxygen and moisture can be achieved by encapsulation of the device. The encapsulation can be obtained by means of a single thin-film layer situated on the substrate, surrounding the OLED.
Currently, OLED display technology utilizes two general approaches to deliver color images. The first approach is a color display where three entirely different OLED stacks reside on three adjacent individually driven sub-pixels to produce red, green, and blue (RGB) color emissions due to distinct configuration of organic layers within the individually designed stacks. This approach is appropriate for large size displays, but has certain difficulties in being implemented in color displays where pixel and sub-pixel dimensions are on the scale of micrometers.
The second approach is a color display where white emission from a common OLED stack is filtered through three single color filter layers residing on three adjacent sub-pixels to produce red, green and blue colors (alternatively referred to herein as RGB colors). This second approach can be comfortably used for both large and small dimension OLED displays.
In the color filter white OLED display of the second approach, each individual filter layer is put on top of a sub-pixel using photolithography technique and possesses distinct function. A red color filter layer transmits only a red portion of a white OLED emission and cutting out or reducing below a level of human perceptibility visible light in the other portions of the visible light spectrum. A green color filter layer transmits only a green portion of a white emission. A blue color filter layer transmits only a blue portion of white emission, preventing red and green emissions from contributing to the image. Thus the chromaticity of such a display using a white OLED stack and three single color filters on three RGB sub-pixels largely depends on the filter layer's ability to transmit or stop certain portions of the white emission in the visible optical band.
Two factors play major role in the transmissivity of color filter material. First, the chemical and physical compositions of the layer are significant, and second, the thickness of the layer is also significant. The layer thickness is important from a display color saturation standpoint, in that the thicker the color filter layer is, the better the performance of the filter layer. However, it is not always possible to attain the desired thickness of a color filter layer due to the limitations in photolithography techniques, spin coating processes, and the properties of the color filter material. It may not be possible to achieve a desired transmittance simply by reducing or increasing a thickness of a layer due to the limits on varying a thickness of a spin coated film.
Color balancing a display requires that the relative transmittance of the three sub-pixels in an RGB display have comparable transmittances. In particular, the combination of the light emitted from an RGB display with all three sub-pixels on should be close to a standard white color, for instance point D65 on a 1931 CIE color coordinate graph.
One of the three color filter materials, for instance a green color filter material, may have a higher transmittance than the other two color filter materials, thereby shifting a white balance for the OLED, or other light emitting device, display. A green filter layer blocks red and blue light. If the green filter has a higher transmittance, then the green light will be stronger, causing green shifting when mixing colors for white, RG (red-green) or BG (blue-green).
High resolution active matrix displays may include millions of pixels and sub-pixels that are individually addressed by the drive electronics. Each sub-pixel can have several semiconductor transistors and other IC components. Each OLED may correspond to a pixel or a sub-pixel, and these terms are used interchangeably herein.
Another problem in balancing OLED display colors may arise when the separation between two adjacent pixels or sub-pixels are being reduced to the micrometer or sub-micrometer dimensions. The smaller the separation is between sub-pixels, the larger the probability of an electrical leakage current between adjacent sub-pixels. The smaller the size of the sub-pixels, the higher the density of the electronic circuit elements in the backplane. A variety of technologies are used to fabricate OLED display backplanes including but not limited to single crystal silicon and polysilicon wafers, glass backplanes with layers of transparent conducting films, and flexible organic or inorganic backplanes.
One common obstacle for all the above mentioned technologies is the possibility of increased leakage current with increasing density and decreasing circuit elements size, irrespective of the OLED stack. While leakage current may be very low and random, on the order of nano- or even pica-Amperes, it may still affect performance of an OLED display. For example, even if all green sub-pixels are turned off, and only red sub-pixels are turned on, the leakage current through green sub-pixels may skew the otherwise pure red color emission produced by the red sub-pixels.
Another type of leakage is an optical leakage of light between RGB sub-pixels. One solution to this problem is to implement a black matrix layer, which is an opaque material placed between sub-pixels.
A passive material that operates as a filter has a transmittance spectrum, an absorbance spectrum and a reflectance spectrum. If light is generated within the material, for instance an emissive layer in an OLED, then the material has an emissive spectrum.